1. Field of the Invention
This invention concerns detachable mooring systems for loading and offloading liquid petroleum product oil tankers, floating storage (FSO) vessels, floating production storage and offloading (FPSO) systems, floating vessels for natural gas offloading (for example, a cryogenic liquefied natural gas (LNG) regas import terminal), and LNG transport vessels.
2. Description of the Prior Art
Numerous patents are known that pertain to disconnectable mooring systems, most of which utilize a submerged buoy that can be detachably released from a floating vessel. For example, U.S. Pat. No. 5,651,708 issued to Borseth shows a detachable buoy with a geostationary part. The Borseth buoy has an outer body that is received in a recess in the bottom of the vessel, where the outer body is fixed to the vessel by locking wedges. Four other notable types of detachable mooring systems are known and are illustrated in FIGS. 1 to 4.
FIGS. 1A and 1B illustrate a disconnectable mooring system of a design of FMC Technologies and as illustrated by U.S. Pat. No. 5,240,446. The mooring system has two basic parts: a geostationary buoy 61 and a detachably connectable turret assembly 53 that is disposed in the floating vessel. The buoy 61 is moored to the seabed by a number of anchor legs 63 that are connected to the buoy at anchor leg connectors 62, such that the buoy is generally geostationary when the anchor legs 63 are anchored to the sea floor.
The vessel 52 carries a turret assembly 53, which is rotatively mounted within the vessel hull and which opens to the sea near the keel elevation. The turret 53 includes a vertical turret shaft 59 and is supported by an upper axial bearing 57 and a lower radial bearing 58. The turret and bearings remain on the vessel when the buoy is disconnected therefrom. The lower end of the turret shaft 59 is equipped with a structural connector 60 that is designed and arranged to disengageably connect with a connector hub 66 mounted at the upper surface of the buoy 61. Rubber fenders 64 are provided on the buoy to cushion the mooring process. A water seal 67 is provided to maintain watertight integrity of the turret compartment in the vessel.
The turret mooring arrangement of FIGS. 1A and 1B provides a fluid flow path between a subsea well or component and the vessel when the vessel is moored to the buoy. The fluid transfer system (FTS) 54 includes a flexible conductor 68 spanning the distance between the seabed and the buoy 61, a lower conductor pipe 56a that is geostationary and in fluid communication with the flexible conductor 68, and an upper conductor pipe 56b, which is fixed to the vessel and in fluid communication with the lower conductor pipe 56a via a fluid swivel 55.
When the buoy 61 is completely separated from the vessel 52, the buoy 61 is designed and arranged to sink to a neutrally buoyant position about 36 meters below sea level. As shown in FIG. 1B, the vessel is connected to the buoy by first recovering the submerged buoy upwards to the structural connector 60 by heaving in a retrieval line 65 with a winch system (not shown). The structural connector 60 is then locked in engagement with the connector hub 66, fixing the turret with the geostationary buoy and mooring the vessel 52 to the seabed. The vessel can freely weathervane about the geostationary turret in response to wind, waves and currents.
FIGS. 2A and 2B show a later version of a disconnectable turret mooring arrangement 71 design of FMC Technologies. The turret mooring arrangement 71 of FIGS. 2A and 2B is substantially similar to the turret mooring arrangement 51 of FIGS. 1A and 1B. For example, the buoy 81 is moored to the seabed by a number of anchor legs 83 that are connected to the buoy at anchor leg connectors or cars 82, such that the buoy is generally geostationary. The vessel 72 carries a turret assembly 73, which is revolvably disposed within the vessel hull and which opens to the sea near the keel. The turret assembly 73 includes a vertical turret shaft 79 which is supported by an upper axial bearing 77 and a lower radial bearing 78. The turret and bearings remain on the vessel when the buoy is disconnected. The lower end of the turret shaft 79 is equipped with a structural connector 80 that is designed and arranged to disengageably connect to a connector hub 86 disposed at the upper surface of the buoy 81. A water seal 87 is provided to maintain watertight integrity of the turret compartment in the vessel. The fluid transfer system (FTS) 74 includes a flexible conductor 88 between the seabed and the buoy 81, a lower geostationary conductor pipe 76b in fluid communication with the flexible conductor, and an upper conductor pipe 76a, fixed to the vessel and in fluid communication with the lower conductor pipe 76b via a fluid swivel 75. When the buoy 81 disconnects from the vessel 72, the buoy 81 is of a design so that it sinks to a neutrally buoyant position about 36 meters below sea level. A retrieval line 85 is provided for heaving the buoy to the vessel.
However, unlike the turret mooring arrangement of FIGS. 1A and 1B, where the buoy 61 abuts the keel of the moored vessel 52, in the arrangement of FIGS. 2A and 2B, the upper part of a buoy 81 is cone shaped and is brought into a cone shaped buoy receiving space 89. The structural connector 80 fastens the buoy 81 to the turret shaft 79. The turret shaft 79 is rotatively connected to the vessel 72 by the upper bearing 77. The skirt 90 is rotatively coupled to the lower bearing 78. This system is advantageous when several large fluid conductors 88 are required.
FIGS. 3A and 3B generally describe a subsurface buoy mooring system 101 such as that shown by Svensen in U.S. Pat. No. 4,892,495. A cone-shaped buoy 103 is rotatively received into a receptacle 108 formed in the vessel hull 111 and is secured inside a complementary turret receptacle 104 by latches 105. A radial bearing 106 and a vertically-oriented axial bearing 107 support turret 102. The axial bearing 107 abuts a bearing support surface 110. When the buoy 103 is disconnected from the vessel, the turret and the bearings remain on the vessel. The buoy 103 is moored to the seabed by a number of anchor legs 109 such that the buoy is essentially geostationary. For simplicity, the fluid transfer system is not illustrated.
FIGS. 4A and 4B illustrate a type of mooring system 121 design of Advanced Production Loading (APL) AS of Norway and described in U.S. Pat. No. 5,468,166, among others. A buoy assembly 124 includes a buoy 128, upper and lower bearings 126, 127, and a turret 125 that is rotatably supported by the bearings. The cone-shaped buoy 128 is non-rotatably secured into a complementary receptacle 137 formed in the vessel hull 122 by latches 134 that engage a groove 135 formed in the buoy.
The fluid transfer system (FTS) includes a flexible conductor 133 spanning the distance between the seabed and the buoy 128, a lower conductor pipe 132 that is geostationary and in fluid communication with the flexible conductor, and an upper conductor pipe 136, which is fixed to the vessel and in fluid communication with the lower conductor pipe 132 via a fluid swivel 123.
However, the buoy 128 is not geostationary. The buoy is attached to and rotates with the vessel hull 122 while the turret 125 remains geostationary. When the buoy assembly 124 is disconnected from the vessel 122, the bearings 126, 127 and the turret 125 remain on the buoy. The lower end of the turret 125 includes a chain table or anchor leg frame 129 with anchor leg connectors or ears 131. A number of anchor legs 130 connect the chain table 129 and turret 125 to the seabed so that the turret 125 is essentially geostationary. In this design the entire anchor leg system weight and loads are supported by the axial bearing 126. Because the buoy 128 rotates, it does not serve to reduce vertical bearing loads.
Most mooring systems are “turret” systems of one form or another which are familiar to the art of mooring design. Turrets are generally large and expensive structures that usually include large diameter upper and lower bearings. Many prior art disconnectable mooring systems also require a large (approximately 10 meters diameter or larger cone shaped opening in the vessel bottom. Such structure mandates expensive vessel construction.
Accordingly there is a need for a new design to reduce the cost of mooring structures. Furthermore, large openings in the vessel hull to accommodate mooring buoys cause significant drag and energy losses on those disconnectable cargo vessels required to said long distances. Because newer and larger high speed LNG carrier/regas vessels tend to have a narrow flat bottom near the bow at the optimum location for a buoy connection, a large hull opening is a less desirable in these applications.
3. Identification of Objects of the Invention
A primary object of this invention is to provide a detachable mooring system that does not require a turret for connection between a vessel and a mooring buoy, but rather provides a connector flange between a vessel mounted hydraulic connector and the buoy, with an axial/radial bearing assembly between the buoy and a chain table secured to the sea floor.
Another object of this invention is to provide a detachable mooring system having a buoy supported on a chain table with a bearing assembly with a relatively large radial dimension as compared to prior art arrangements so that a large radial mooring load capacity is achieved. Detachable moorings having larger radial load capacity are desirable because hydrocarbon production and import/export terminals are required in more hostile environments than in the past.
Another object of the invention is to provide a mooring system that requires a significantly smaller opening in the vessel that includes the capability to plug the opening so that a virtually smooth ship bottom is achieved at the buoy connection point.
Another object of the invention is to provide an improved detachable mooring system including buoy-to-ship interface equipment that can be released and recovered in high sea states and harsh conditions.